The invention relates to an attenuation element.
Traction converters for electric locomotives frequently have converters with a plurality of distributed voltage intermediate circuits. An equivalent circuit diagram for such a traction converter is shown in more detail in FIG. 1. In this equivalent circuit diagram, each converter 2 or 4 has a grid-side and a load-side converter 6 and 8. The grid-side converter 6 provided is what is known as a four-quadrant controller (4QS), with the load-side converter 8 provided being a self-commutated pulse-controlled converter. On the DC voltage side, these two converters 6 and 8 in each converter 2 or 4 of the traction converter are connected in electrical parallel by means of an intermediate circuit capacitor 10 or 12 which may be designed from a multiplicity of capacitors. These intermediate circuit capacitors 10 and 12 of the traction converter are electrically connected to one another and to DC-voltage-side connections of each converter 2 and 4 of the traction converter by means of electrical connections 14 and 16, particularly busbars. These intermediate circuit capacitors 10 and 12 have an auxiliaries converter 18 connected in electrical parallel with them. In addition, there is a circuit breaker 22 in a connecting line 20 for the auxiliaries converter 18. The DC-voltage-side connections of the auxiliaries converter 18 have a capacitor 24 connected in electrical parallel with them. Likewise, the intermediate circuit capacitors 10 and 12 of the converters 6 and 8 have an acceptor circuit 26 connected in electrical parallel with them which is designed as a series resonant circuit and is attuned to the harmonics of the intermediate circuit. To this end, this acceptor circuit 26 has a capacitor 28, inter alia. This equivalent circuit diagram likewise reveals that the connection 14 between the two converters 2 and 4 of the traction converter can be isolated such that the acceptor circuit 26 remains connected in electrical parallel with the capacitor 10 or 12 of the converter 2 or 4. For this reason, two circuit breakers 30 and 32 are arranged in the connection 14.
The electrical connections 14 and 16, particularly busbars, have a direct current flowing through them for power transfer between the voltage intermediate circuits of the two converters 2 and 4 of the traction converter. Since the grid-side converter 6 of each converter 2 and 4 of the traction converter is in single-phase form, a current at twice the grid frequency additionally flows in the electrical connections 14 and 16. The leakage inductances of the electrical connections 14 and 16 result in a higher-order resonant circuit in conjunction with the intermediate circuit capacitors 10 and 12 and the capacitors 24 and 28. This resonant circuit is excited upon each switching operation by a converter valve in a converter 2 or 4 on the voltage intermediate circuit of the traction converter and results in lightly attenuated, high-frequency currents between the voltage intermediate circuits. These currents result in supplementary thermal loading of the intermediate circuit capacitors 10 and 12 and of the capacitor 24 in the auxiliaries converter 18.
FIG. 2 uses a graph to show individual capacitor currents iC2, iC4, iCH and iCS over frequency. This illustration reveals that in all capacitors 10, 12, 24 and 28 of the converters 2, 4 and 18 and of the acceptor circuit 26, current gain takes place when the higher-order resonant circuit is excited. At a frequency below a resonant frequency, the frequency components are evenly split over these capacitors 10, 12, 14 and 28. At the resonant frequency, the capacitor currents iC2, iC4, iCH and iCS are amplified, and at a frequency above the resonant frequency the capacitor currents iC2, iC4, iCH and iCS are affected by the excitation differently.
EP 1 450 475 A1 discloses a measure which can be used to attenuate oscillations in the voltage intermediate circuit of a voltage intermediate circuit converter. This measure is an attenuation network which is connected in electrical parallel with each energy store of this voltage intermediate circuit converter. This attenuation network comprises a capacitor and a resistor which is connected in series with this capacitor. To achieve a good action by this attenuation network, the connections to the intermediate circuit capacitor are in low-inductance form. To dissipate a power loss, the resistor in this attenuation network preferably has a separate resistor cooling system. In addition, it is advantageous if the value of the capacitor in the attenuation network is in the order of magnitude of 1.5 to 50 times greater than the value of the capacitor in the voltage intermediate circuit. This attenuation network can be used to reduce the oscillation by the current in the oscillation path, and to reduce the electrical and thermal loading on the capacitors and on further components of the voltage intermediate circuit converter.
FIG. 3 shows a more detailed illustration of the equivalent circuit diagram for the traction converter shown in FIG. 1 with two attenuation networks 34 and 36 from EP 1 450 475 A1. According to EP 1 450 475 A1, an attenuation network 34 or 36 needs to be connected in electrical parallel with the intermediate circuit capacitor of a voltage intermediate circuit converter. The capacitor 10 or 12 has the attenuation network 34 or 36 connected in electrical parallel with it. The attenuation network 34 or 36 has a capacitor 38 or 40 and a resistor 42 or 44 which are connected in electrical series.
FIG. 4 uses a graph to show the capacitor currents iC2, iC4, iCH and iCS over the frequency f. To be able to recognize the action of the two attenuation networks 34 and 36, the current profiles in FIG. 2 and the attenuated current profiles are shown together in this graph. The dashed profiles illustrate the current profiles iC2, iC4, iCH and iCS with attenuation. At frequencies f below the resonant frequency, these attenuation networks 34 and 36 exhibit no action. The action of attenuation networks 34 and 36 is also limited at the resonant frequency.